The present invention relates to a cryogenic process to produce nitrogen at elevated pressure and oxygen, where nitrogen recovery is high, typically greater than 70%, preferably greater than 85%, and oxygen recovery is significantly less than 100%, typically less than 70% and preferably less than 55%. In certain industrial applications, i.e., the electronics or petrochemical industry, there is a need for nitrogen and, sometimes, a small amount of oxygen. The complete separation of nitrogen and oxygen from an air feed (from a full recovery plant) would be highly inefficient when there is no market for the produced oxygen in excess of the required oxygen. Therefore, there is a need for an efficient air separation plant with a high nitrogen recovery and a relatively low oxygen recovery.
There are several processes in the art for the production of nitrogen, but very few relate to processes where small quantities of oxygen are simultaneously coproduced.
Nitrogen generators may consist of one, two or more distillation columns. The improvement of the present invention relates to nitrogen generators consisting of two or more columns.
In a double column nitrogen generator, each of the columns can be a full size distillation column or it can be reduced to a smaller fractionator containing as few as one fractionation stage (in addition to a reboiler or condenser, if applicable).
U.S. Pat. No. 4,604,117 teaches a cycle consisting of a single column with a prefractionator that creates new feeds (of different compositions) to the main column.
U.S. Pat. Nos. 4,848,996 and 4,927,441 each teach a nitrogen generator cycle with a post-fractionator. The post-fractionator, which is thermally integrated with the top of the rectifier, separates oxygen-enriched bottom liquid into even an more oxygen-enriched fluid and a vapor stream with a composition similar to air. This "synthetic air" stream is then warmed, compressed and recycled back to the rectifier.
U.S. Pat. No. 4,222,756 teaches a classic double column process cycle for nitrogen production. In the classic double column cycle, the objective of the first (higher pressure) column is to separate feed air into a nitrogen overhead vapor and an oxygen-enriched liquid that is subsequently processed in the second column (usually operated at a lower pressure) to further recover nitrogen.
GB Patent 1,215,377 and U.S. Pat. Nos. 4,453,957; 4,439,220; 4,617,036; 5,006,139 and 5,098,457 teach various other embodiment of a double column nitrogen generator. The concepts taught in these patents vary in the means of thermal integration of columns, e.g., using different media in reboilers/condensers and applications of intermediate or side reboilers in the columns. Other differences are in the means of supplying refrigeration to the plant, e.g., by expansion of different media.
U.S. Pat. No. 4,717,410 teaches another double column nitrogen generator process schemes. In this taught generator, the recovery of a high pressure nitrogen product is increased (at the expense of the recovery of the lower pressure nitrogen) by pumping back liquid nitrogen from the lower pressure column to the higher pressure column.
U.S. Pat. Nos. 5,069,699; 5,402,647 and 5,697,229, as well as, EP 0701099 each teach nitrogen generators schemes which contain more than two columns. The additional column or a section of a column is used either to further increase the recovery and/or the pressure of nitrogen product or to provide an ultra high purity nitrogen product.
U.S. Pat. No. 5,129,932 teaches a cryogenic process for the production of moderate pressure nitrogen together with a high recovery of oxygen and argon. The increase in nitrogen pressure, in comparison with the art referenced above, is achieved by expanding a portion of nitrogen from the high pressure column, however, the process is a full recovery cycle.
U.S. Pat. No. 5,049,173 teaches the principle of producing ultra high purity oxygen from any cryogenic air separation plant. In particular, the improvement comprises removing an oxygen-containing but heavy contaminant-free stream from one of the distillation columns and further stripping this stream from light contaminants in a fractionator to produce ultra high purity oxygen. The heavy contaminant-free stream is obtained by withdrawing the stream from a position above the heavy contaminant-containing feed(s).
U.S. Pat. No. 4,448,595 teaches the use of a double column air separation process, where boilup for the lower pressure column is supplied by a portion of a feed air (a "split column"), to produce nitrogen and, optionally, some oxygen. All the oxygen product is produced from the lower pressure column along with at least some of the nitrogen product. The oxygen product is withdrawn from (or near) the bottom of the lower pressure column as liquid and then vaporized at the top of this column. If the purity of the oxygen product is greater than 97%, the patent teaches that the product can be withdrawn from the bottom of the low pressure column. Any excess oxygen may be withdrawn from the lower pressure column in a waste stream. This waste stream contains also nitrogen which reduces significantly nitrogen recovery from this column. The improvement of this patented invention manifests itself in that the lower pressure column operates at elevated pressure, providing nitrogen product at elevated pressure. Therefore, the waste stream contains excess pressure energy and is expanded to provide the necessary refrigeration for the plant. If the refrigeration is provided by other means (e.g., a liquefier), the waste expander is no longer necessary and can be eliminated.
Single column nitrogen generators are not relevant to the process of the present invention, because they are unable to provide a high recovery of nitrogen. Nevertheless, to provide a more complete review of the background art, the patents teaching single column nitrogen generator cycles are provided.
U.S. Pat. Nos. 4,560,397 and 4,783,210 each teach process schemes for the coproduction of oxygen using a single column nitrogen generator.
U.S. Pat. No. 4,560,397 teaches a process for the production of elevated pressure nitrogen, together with ultra high purity oxygen. In this process, a two-column cycle is used, where the first, higher pressure, column is devoted to nitrogen production and the oxygen product is withdrawn from the second, lower pressure, column, at a point above the liquid sump, to avoid heavy impurities.
U.S. Pat. No. 4,783,210 teaches a single column nitrogen generator where an oxygen-enriched liquid from the bottom of the nitrogen generator is partially boiled in a reboiler-condenser on top of the nitrogen generator, resulting in a vapor waste stream, and in a second oxygen-enriched liquid that is eventually purified in an additional column.